Difficulties monitoring short-term ageing in thin surfacing layers using asphalt concrete

Ageing has a profound effect on pavement performance, especially regarding cracking. Due to budgetary constraints, South Africa has pioneered the use of thin asphalt concrete layers. The research described in this paper is based on a short-term ageing study in South Africa, using data generated over a period of 6 years. During this time, polymer modified asphalt binders were increasingly employed in road construction, and rheological analyses from the dynamic shear rheometer were increasingly used to characterize asphalt binders. This study compared the complex shear modulus to the softening point as an ageing index property used to monitor the extent of short-term ageing of the recovered asphalt binder from newly laid asphalt concrete. The asphalt binder properties from 20 constructions sites were evaluated, whereby the recovered binder from the site shortly after construction was evaluated against the asphalt binder properties obtained in the laboratory after the rolling thin film oven treatment. The results indicate that the recovery process leads to a deterioration in the repeatability for the complex shear moduli obtained from recovered asphalt binders. The lower repeatability prevents meaningful conclusions from being made. Furthermore, the work shows that although the Rolling Thin Film Oven Test may be a good predictor of short-term ageing when using softening point as an ageing index property, it is only valid for unmodified asphalt binders in South Africa.